Technical Abstract:
Iron is essential for plants; Fe-deficiency chlorosis occurs in many regions where susceptible crops are grown, or where potentially toxic metals have accumulated in acidic soils. Such metals induce Fe-deficiency in both dicots and Poaceae species, but the mechanism appears to be different because of the method these species use to accumulate soil Fe. For Poaceae, Ni, Cu, and Zn can form chelates with phytosiderophores, can inhibit dissolution of soil Fe, and can displace Fe from exocellular phytosiderophores, thereby interfering with Fe dissolution and uptake. If ferric-phytosiderophores are not formed in the rhizosphere, chlorosis of Poaceae will result. On the other hand, additions of hydrous Fe oxides can improve metal adsorption by soils and reduce metal uptake (presume unchanged pH). We have used Fe additions to reduce Ni phytotoxicity in smelter contaminated soils; Fe and limestone together induced Mn deficiency in organic soils, but when Mn was also supplied, soil metals were effectively remediated. Metal toxicity is prevented by raising soil pH. Details of metals speciation in relation to Ni-induced Fe-deficiency will be presented. In general, Poaceae were more resistant to soil Ni than dicots, and Ni inhibited both uptake and translocation of Fe by soybean. Another role of plant Fe has been found to be critical to Cd risk to humans when subsistence rice farmers consume home grown rice. Rice is such a poor supply of Fe and Zn that Cd absorption is greatly increased compared to other kinds of diets. Fe deficiency in particular causes increased Cd absorption and retention in the small intestine. Thus Fe and metals interact from soil chemistry to plant physiology to human nutrition and these interactions should be considered when one attempts to diagnose and solve plant nutrition and environmental metals problems.